The present invention relates to methods and apparatuses for evaluating conditions of coatings on metallic substrates, more particularly to such methods and apparatuses involving electrochemical noise.
Electrochemical impedance spectroscopy (EIS) is an electrochemical methodology in which an ac signal (typically, a small voltage signal) is applied to an electrode (e.g., a corroding metal) and the response is measured. The current-time and the voltage-time measurements are processed to provide a representation of the ac impedance at different frequencies, known as the xe2x80x9cimpedance spectrum.xe2x80x9d
The term xe2x80x9cimpedancexe2x80x9d is the ac analogue of dc resistance. The relationship for dc is given by Ohm""s law, V=IR, wherein V (e.g., in volts) is the voltage across a resistor R (e.g., in ohms) and I (e.g., in amps) is the current. Similarly, the relationship for ac is given by V=IZ, wherein Z is the impedance of the circuit. Unlike resistance R, impedance Z may depend on the frequency f (e.g., in hertz, which is the number of cycles per second) of the applied ac signal.
Two parameters which relate the output current to the input voltage define the impedance of a system at a given frequency. The first parameter is the amplitude of the ac current divided by the amplitude of the ac voltage. The second parameter is the phase angle, which is proportional to the shift in time between peak current and peak voltage. The impedance spectrum comprises an accumulation of values of these parameters for various frequencies.
In the past teen to twenty years, electrochemical impedance spectroscopy (EIS) has become widely accepted as a nondestructive technique for evaluating the electrochemical properties of non-conductive coatings applied to metallic substrates. EIS data (such as maximum impedance, Zmax) have been successfully equated to given coating conditions. For example, a Zmax value of 109xe2x88x921011 ohms-cm2 indicates a xe2x80x98goodxe2x80x99 coating while a Zmax value less than 106 ohms-cm2 indicates a xe2x80x98badxe2x80x99 coating.
U.S. Navy researchers have used EIS in the laboratory to characterize and evaluate many organic coating systems on metallic substrates for long periods (up to 10 years) of exposure to saltwater. Due to the logistics of the test method, however, it is not practical to perform EIS testing in the field to evaluate the condition of coatings on ships and vehicles. While EIS can be performed in the field, it requires a relatively large amount of time to perform each test, and the data are somewhat complicated. Efforts to make in-field EIS testing more logistically feasible have shown varying degrees of success, but the lack of straightforward, easy-to-interpret output from EIS tests remains a hindrance to the widespread use of EIS testing as a monitoring technique in the field.
Electrochemical noise (ECN), also referred to as electrochemical noise analysis (ENA) or electrochemical noise measurement/method(s) (ENM), is a nondestructive analysis technique in which the direct, current xe2x80x9cnoisexe2x80x9d and voltage xe2x80x9cnoisexe2x80x9d associated with electrochemical reactions on a metallic surface are each measured and recorded. The meaning of the word xe2x80x9cnoisexe2x80x9d in the context of ECN is distinguishable from its commonly understood meaning, wherein the word xe2x80x9cnoisexe2x80x9d refers to unwanted sound. Electrochemical noise does not involve audible sounds (i.e., fluctuations in air pressure or acoustic noise), but rather is concerned with fluctuations in electrochemical potential and electrochemical current. Electrochemical potential noise is the fluctuation in the electrochemical potential of an electrode relative to a reference electrode. Electrochemical current noise is the fluctuation in an electrochemical current.
Generally, measurement of ECN involves the utilization of three test electrodes. For instance, two steel electrodes are connected to an ampmeter, and current therebetween is recorded; one of the two steel electrodes and a reference electrode are connected to a voltmeter, and voltage therebetween is recorded. Although only one of the two steel electrodes is connected to the voltage meter, the two steel electrodes effectively behave as a single electrode of twice the area, since the ammeter used to measure current is assumed to behave ideally (i.e., measuring current with no voltage drop). While the three test electrodes are immersed in a salt solution, two kinds of xe2x80x9ctime recordsxe2x80x9d are effectuated, viz., current-against-time (variation of current with time) and potential-against-time (variation of electode potential with time).
ECN testing has been used in the past ten years to evaluate the kinetics of localized electrochemical reactions and processes, such as pitting reactions on passive alloys. EIS has been much more commonly effectuated than has ECN for evaluating coating conditions. More recently, ECN has been gaining interest as a technique for evaluating coatings, albeit that EIS testing remains the more xe2x80x9ctried-and-true,xe2x80x9d traditional approach for such purposes.
For instructive discussion regarding EIS and ECS in relation to the electrochemistry of corroding metal samples, see Robert Cottis and Stephen Turgoose, Electrochemical Impedance and Noise (Corrosion Testing Made Easy), NACE International, 1440 South Creek Drive, Houston, Tex. 77084, 1999, incorporated herein by reference; see, especially, Chapter 1, pages 1-7.
Also incorporated herein by reference are the following articles: John N. Murray, xe2x80x9cEvaluation of Electrochemical Noise to Monitor Corrosion for Double Hull Applications,xe2x80x9d Technical Report, Naval Surface Warfare Center, Carderock Division, CARDIVNSWC-TR-61-94/29, August 1994; Gordon P. Bierwagen, Carol S. Jeffcoate, Junping Li, Seva Balbyshev, Dennis E. Tallman, Dougals J. Mills, xe2x80x9cThe Use of Electrochemical Noise Methods (ENM) to Study Thick, High Impedance Coatings,xe2x80x9d Progress in Organic Coatings 29, 1996, pp 21-29; Colin J. Sandwith and Robert L. Ruedisueli, xe2x80x9cCorrosion and Aging Testsxe2x80x94Via Measurements of Insulation Resistance, Impedance, and Electrochemical Noisexe2x80x94on Jackets of Small-Diameter, Armored, Fiber-Optic Cables with and without Simulated Biofouling Damage,xe2x80x9d. Proceedings of the Ocean Community Conference 1998, Marine Technology Society, Baltimore, Md., Nov. 16-19, 1998, pp393-397; Gretchen A. Jacobson; Managing Editor, xe2x80x9cCorrosion Control,xe2x80x9d Materials Performance, January 2000, pp 22-27; Jeffery R. Kearns, John R. Scully, Pierre R. Roberge; David L. Reichert, John L. Dawson, Eds., xe2x80x9cOverview,xe2x80x9d pp ix-xvii, Electrochemical Noise Measurement for Corrosion Applications, ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994; David L. Reichert, xe2x80x9cElectrochemical Noise Measurement for Determining Corrosion Rates,xe2x80x9d Electrochemical Noise Measurement for Corrosion Applications, Jeffery R. Kearns, John R. Scully, Pierre R. Roberge, David L. Reichert, John L. Dawson, Eds., ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994, pp 79-89; Gordon P. Bierwagen, Douglas J. Mills, Dennis E. Tallman, Brian S. Skerry, xe2x80x9cReproducibility of Electrochemical Noise Data from Coated Metal Systems,xe2x80x9d Electrochemical Noise Measurement for Corrosion Applications, Jeffery R. Kearns, John R. Scully, Pierre R. Roberge, David L. Reichert, John L. Dawson, Eds., ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994, pp 427-445; John N. Murray, xe2x80x9cElectrochemical Test Methods for Evaluating Organic Coatings on Metals: An update. Part I. Introduction and Generalities Regarding Electrochemical Testing of Organic Coatings,xe2x80x9d Reprinted from Progress in Organic Coatings 30, 1997, pp 225-233; John N. Murray, xe2x80x9cElectrochemical Test Methods for Evaluating Organic Coatings on Metals: An update. Part III. Multiple Test Parameter Measurements,xe2x80x9d Reprinted from Progress in Organic Coatings 31, 1997, pp 375-391; Gordon Bierwagen, Douglas J. Mills, xe2x80x9cCharacterization of Corrosion under Marine Coatings by Electrochemical Noise Methods,xe2x80x9d Final Report for the period Sep. 1, 1992-Aug. 30, 1994, Grant Number N 00014-93-1-0013, The Office of Naval Research, 800 N. Quincy Street, Arlington, Va. 22217-5660 (61 pp plus cover page, errata page); F. Mansfeld, L. T. Han, C. C. Lee, xe2x80x9cAnalysis of Electrochemical Noise Data for Polymer Coated Steel in the Time and Frequency Domains,xe2x80x9d J. Electrochem. Soc., Vol. 143, No. 12, December 1996, pp L286-L289; Gordon Bierwaen, Junping Li, Seva Balbyshev, Jason Lindquist, xe2x80x9cElectrochemical Noise Methods Applied to the Study of Organic Coatings,xe2x80x9d Final Office of Naval Research Report, Grant No. N00014-95-1-0507, July 2000, Department of Polymers and Coatings and Department of Chemistry, North Dakota State University, Fargo, N.Dak. 58105 (191 pages).
Further incorporated herein by reference are the following United States patents: Pope et al. U.S. Pat. No. 5,888,374 issued Mar. 30, 1999; Murray U.S. Pat. No. 5,746,905 issued May 5, 1998; Shih et al. U.S. Pat. No. 5,373,734 issued Dec. 20, 1994; Berg et al. U.S. Pat. No. 5,236,564 issued Aug. 17, 1993; Baer et al. U.S. Pat. No. 5,093,626 issued Mar. 3, 1992; Kihira et al. U.S. Pat. No. 4,806,849 issued Feb. 21, 1989; Hladky U.S. Pat. No. 4,575,678 issued Mar. 11, 1986; Mansfeld et al. U.S. Pat. No. 4,221,651 issued Sep. 9, 1980.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for performing on-site (e.g., in-field or in-service) evaluations of coatings on metallic substrates.
The present invention provides a device, system and method for evaluating coating condition using electrochemical noise (ECN) and a xe2x80x9cwitnessxe2x80x9d (e.g., steel or platinum) specimen. According to this invention, the witness specimen is a bare (uncoated) electrode and contacts (e.g., is immersed in) the same (identically contained) electrolyte solution as do a standard xe2x80x9creferencexe2x80x9d electrode and the coated metallic substrate (such as a coated test panel, a tank wall area or a ship""hull area), thus obviating the need for a conventional xe2x80x9csalt bridgexe2x80x9d between plural discretely contained electrolyte quantities.
In accordance with many embodiments of the present invention, coating assessment apparatus is provided which is suitable for on-site use in association with a coated metal substrate and with electrochemical noise instrumentation including an ammeter (which measures current) and a voltmeter (which measures voltage). The inventive apparatus comprises a receptacle, a witness electrode, a reference electrode and lead means. The receptacle is for containing electrolyte and for being coupled with the coated metal substrate whereby the electrolyte contacts the coated metal substrate. The witness electrode is for contacting the electrolyte and for connecting to the ammeter. The reference electrode is for contacting the electrolyte and for connecting to the voltmeter. The lead means is for connecting a metal portion of the coated metal substrate to the ammeter and the voltmeter.
Further provided by many embodiments of the present invention is a system, based on electrochemical noise, for evaluating the condition of a coating on a metallic substrate. The inventive system comprises an ammeter, a voltmeter, an electrolyte reservoir, a witness electrode, a reference electrode, a first conductor, a second conductor and a third conductor. The electrolyte reservoir is attachable to the metallic substrate whereby the electrolyte communicates with the metallic substrate. The witness electrode is contactable with the electrolyte (e.g., communicable with a liquid electrolyte-containing sponge-like member placed inside the reservoir; or, immersible in the reservoir which contains free-standing liquid electrolyte). The reference electrode is contactable with the electrolyte (e.g., communicable with a liquid electrolyte-containing sponge-like member placed inside the reservoir; or, immersible in the reservoir which contains free-standing liquid electrolyte). The first conductor is for connecting the metallic substrate with the ammeter and with to the voltmeter. The second conductor is for connecting the witness electrode with the ammeter. The third conductor is for connecting the reference electrode with the voltmeter.
Also provided by many embodiments of the present invention is a method, based on electrochemical noise, for evaluating the condition of a coating on a metallic substrate. The inventive method comprises the steps of: attaching an electrolyte reservoir to the metallic substrate whereby the electrolyte communicates with the metallic substrate; causing a witness electrode to contact the electrolyte (e.g., immersing the witness electrode in the electrolyte reservoir; or, causing the witness electrode to touch a sponge-like member placed inside the reservoir); causing a reference electrode to contact the electrolyte (e.g., immersing the reference electrode in the electrolyte reservoir; or, causing the reference electrode to touch a sponge-like member placed inside the reservoir); electrically connecting the metallic substrate with an ammeter and with a voltmeter; electrically connecting the witness electrode with the ammeter; and, electrically connecting the reference electrode with the voltmeter.
In accordance with typical embodiments of the present invention, a device for using electrochemical noise analysis for purposes of assessing the coating upon a conductive (e.g., metal) substrate comprises an electrolyte-containing vessel, a witness electrode,sa reference electrode and a working conductor. The electrolyte-containing vessel is adaptable to being firmly coupled with the coated conductive substrate so that the electrolyte is adjacent to a coated region of the metal substrate located inside the electrolyte-containing vessel. The witness electrode is contactable with (e.g., immersible in) the electrolyte and is adaptable to being connected to an ammeter. The reference electrode is contactable with (e.g., immersible in) the electrolyte and is adaptable to being connected to a voltmeter. The working conductor is adaptable to connecting the ammeter and the voltmeter to a noncoated region of the metal substrate located outside the electrolyte-containing vessel.
In inventive principle, the region of the coated conductive substrate which is contacting the electrolyte and is bounded by an electrolytic contact-permitting opening in the electrolyte-containing vessel is effectively rendered a working electrode. In situ application can be propitiously afforded by fixing and sealing the bottom of the device with respect to a coated conductive substrate such as would be part of a marine or nonmarine transportation vehicle or a tank wall. If the vessel is infused with free-standing liquid electrolyte, a top seal is preferably provided. If an electrolyte-soaked sponge or foam member is disposed within the vessel, a top seal may be optional.
Inventive practice will normally prescribe that the same electrolytic entity (e.g., a liquid electrolyte-saturated sponge disposed inside a cell; or, a body of electrolyte liquid contained by a cell) will be in contact with the witness electrode, the reference electrode and the coated metallic substrate test region (e.g., the portion of the metallic substrate which is bordered by the perimeter of the base opening of the cell and is adjacent to the electrolytic entity).
According to typical inventive embodiments, a generally cylindrical electrolyte reservoir (such as includes a vessel, container, chamber, receptacle, cell, etc.) is capable of being coupled with the coated metallic substrate so that the electrolyte is contiguous with the coated metallic substrate through the open lower end of the reservoir. A first conductor means is situated outside the reservoir and connects an uncoated portion of the metallic substrate (e.g., a bare spot, a projection or a stud) to an ammeter and a voltmeter; that is, the first conductor means is in electrical contact with the uncoated metallic substrate portion, and with the ammeter, and with the voltmeter. A second conductor means passes through the upper end of the reservoir and connects the witness electrode to the ammeter; that is, the second conductor means is in electrical contact with the witness electrode and with the ammeter. A third conductor means passes through the upper end of the reservoir and connects the reference electrode to the voltmeter; that is, the third conductor means is in electrical contact with the reference electrode and with the voltmeter. The coated metallic substrate portion which is in electrical contact with the electrolyte and is inside the periphery of the lower open end of the reservoir functionally represents the working electrode; in other words, this electrically connected (coated) metallic substrate portion effectively acts as a working electrode.
According to many inventive embodiments, the chamber will be provided with a first opening means (including at least one opening) for allowing the leads to enter the chamber, and a second opening means (including at least one opening) for allowing the electrodes to contact the substrate. Typically, the first opening means (for permitting lead entry) and the second opening means (for permitting electrode contact) will be located at generally or approximately opposite extremes of the chamber. As an example, if a cap-seal-type sealing device is implemented for sealing the chamber at its lead-entry end, the cap-seal should be appropriately apertured with one or more openings for accommodating the passing therethrough of the leads; for instance, according to some embodiments a cap-seal has two small holes for correspondingly accommodating the passing therethrough of two leads.
This invention can be practiced in the field (e.g., on ships) to evaluate coating condition. Furthermore, the data obtained using the inventive sensor and method have been demonstrated by the inventors to correlate well with data obtained unsing electrochemical impedance spectroscopy (EIS). It is noted that the U.S. Navy bases its databases pertaining to long-term coating performance on, inter alia, EIS methodology. The U.S. Navy also bases such databases on salt fog spray chamber tests, field exposure tests, visual evaluations, scratch and prohesion. Based on inventive testing, in situ (e.g., in-the-field, or in-service) coating assessments effectuated using inventive ECN sensors are demonstrably relatable to EIS laboratory databases, and may be relatable to other databases, as well.
The present invention uniquely features the performance of ECN tests between a coated steel specimen and a bare steel specimen, which serves as a probe or xe2x80x9cwitnessxe2x80x9d specimen. This inventive ECN approach offers numerous advantages not only over conventional ECN approaches but also over conventional EIS approaches. ECN offers a logistically feasible means of evaluating coating condition in the field, with shorter test times than EIS and with test output that is more straightforward and easier to interpret than EIS data. However, long-term ECN databases on Naval and Department of Defense (DoD) coatings do not exist. By correlating EIS and ECN data, ECN offers a practical means of bridging the gap between laboratory-based data bases and in-situ, condition-based monitoring in the field. The correlation of data from the two techniques (EIS and ECN) offers practical and useful benefits for the evaluation of coating condition and prediction of coating life.
As described herein (including in Appendix A), the inventors investigated a practical methodology for correlating test results of the inventive ECN sensor and procedure with electrochemical impedance spectroscopy (EIS) data. This investigation was conducted at the Naval Surface Warfare Center, Carderock Division (NSWCCD) located in West Bethesda, Md.
ECN measurements obtained using the inventive method can serve as a practical link between existing coatings data bases (which are based on samples tested using EIS throughout long periods of exposure) and inventively acquired data pertaining to in-service coatings performance on ships and vehicles. ECN data from the apparatus and method according to this invention can be correlated with existing EIS data in order to demonstrate that ECN can quantitatively distinguish between xe2x80x9cgoodxe2x80x9d coatings and xe2x80x9cbadxe2x80x9d coatings. The inventors have demonstrated the ability of the inventive ECN system to quantitatively differentiate between xe2x80x98goodxe2x80x99 and xe2x80x98badxe2x80x99 coatings, and thus have as demonstrated the feasibility of using the inventive ECN system to quantitatively probe the condition of ships and vehicles in the field and predict coating life based on existing data bases.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The following appendices are hereby made a part of this disclosure:
Attached hereto marked APPENDIX A and incorporated herein by reference is the following 19-page high school science project report, authored by joint inventor Brian D. Layer, which discloses various aspects of the present invention: Brian D. Layer, xe2x80x9cThe Uses of Electrochemical Noise in the Evaluation of Coating Condition,xe2x80x9d displayed at the science fair which took place at the Poolesville High School science fair on Jan. 15, 2000 in Poolesville Md.